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5.1 Principles 5.8 IN-SITU AIR SPARGING ' In-situ air sparging (IAS) utilizes air injection beneath the water table to volatilize dissolved hydrocarbons in groundwater and force hydrocarbon vapors into the vadose zone, where ' they can be withdrawn by a vapor extraction systems IAS is commonly conducted concurrently with vapor extraction ' IAS induces the transfer of hydrocarbons from the water to air by creating a strong concentration gradient from groundwater to air The vapor extraction wells are surrounded by specifically designed air injection wells that are screened only within the saturated zone A blower connected to the injection wells forces air under pressure into the saturated zone producing air channels that strip dissolved contaminants from the groundwater and nse to the air-water interface Upon reaching the unsaturated zone, the volatilized hydrocarbons ' are drawn into the extraction wells Injection wells are screened within the saturated zone, and normally have short screened ' intervals (5 to 10 feet) Since most of the air escapes from the upper part of the interval where head pressure is the least, little is gained by increasing the length of the screen tinterval The radius of influence of a sparging well is dependent on site geology, the depth of the spaigy large well, and the depth of the scieened interval The IAS pilot test indicated that relativel la radii can be attained at this site (generally sandy silt) g According to Hincliee (1994), "the two mechanisms by which IAS works are volatilization and biodegradation. Volatilization evaporates and then extracts the contaminant Biodegradation typically is stimulated by the introduction of oxygen " ' Measurements of carbon dioxide (CO2) and oxygen (O,) concentrations during the vapor extraction test indicated high concentrations of CO2 and low concentrations of OZ ' Biodegradation typically consumes 02 and produces CO, Measurements collected during the extraction test may indicate that limited biodegradation is occurring at the site The air spas ging and vapor extraction processes will supply O, to the soil and groundwater and ' remove CO2, enhancing biodegredation ' 5.2 Proposed Sparging System The proposed air sparging system will be composed of a blower/compressor and two sparging wells If the SAVE system is chosen, then a compressor will provide air to the ' sparging wells, via subsurface piping If the VACLEEN unit (or similar) is chosen then a blower (EG&G Rotron EN 404 Explosion-Proof Regenerative Blower) will provide air to the sparging wells The low-pressure blower/compressor will provide between 3 and 10 psi 0-1.,-i A.d I Sery e,1'. 13 ' CHAPiN FU.f 431-9.i^105602